A piercing mill pierces and rolls a round billet into a hollow shell. The piercing mill includes two or three inclined rolls provided at equal intervals around a pass line, a pusher provided along the pass line in front of the inclined rolls, and a plug provided on the pass line between the plurality of inclined rolls.
When a round billet is pierced and rolled by the above-described piercing mill, the resulting hollow shell has defects at its inner surface because of the Mannesmann effect. In general, as the billet diameter reduction at plug nose represented by Expression (1) is smaller, such inner surface defects are more restrained.Billet diameter reduction at plug nose(%)=(round billet diameter−roll interval at plug tip end)/round billet diameter×100  (1)
However, if the billet diameter reduction at plug nose is reduced, the round billet is less easily bitten between the plurality of inclined rolls, in other words, so-called defective biting is likely to result.
A technique for reducing such defective biting even if the billet diameter reduction at plug nose is small is disclosed by JP 2006-297400 A.
According to the disclosure, a plurality of skew rollers are provided in front of the inclined rolls and a pinch roller is provided between the plurality of skew rollers and the inclined rolls. The plurality of skew rollers are coupled with a driving source such as a motor and rotated by the driving source to advance a round billet. Furthermore, the pinch roller coupled with the driving source rotates while it holds the round billet, so that the round billet is advanced while being rotated in the circumferential direction. Therefore, if the billet diameter reduction at plug nose is small, the defective biting can be prevented.
However, the force of the pinch roller is not strong enough to push the round billet in contact with the inclined rolls in the axial direction. Therefore, if the billet diameter reduction at plug nose is small, it is highly possible that defective biting is caused. The pinch roller rotates at a fixed circumferential speed by the driving source, while the round billet has its advancing speed greatly changed during the period after it contacts the inclined rolls until it is stably bitten therebetween, and sometimes its advancing speed can be lower than the circumferential speed of the pinch roller. In this way, if the advancing speed of the round billet is different from the circumferential speed of the pinch roller, the pinch roller slips on the surface of the round billet, which results in outer surface defects.
Another technique for restraining defective biting even if the billet diameter reduction at plug nose is small is disclosed by JP 2000-246311 A and JP 2001-162306 A. According to the disclosure of these documents, a round billet is pushed to advance by a pusher and the pusher pushes the round billet in between the inclined rolls. In this case, if the round billet is not bitten between the plurality of the inclined rolls and slips, the pusher pushes the rear end of the round billet to advance and therefore the round billet is eventually pushed in between the inclined rolls. Therefore, defective biting can be prevented.
However, if the billet diameter reduction at plug nose is reduced and the round billet pierced and rolled while it is pushed in between the inclined rolls using the pusher, the inclined rolls are increasingly worn. This is because defective biting is prevented by the pushing force of the pusher and external force applied upon the inclined rolls by the pusher is greater than the case in which the billet diameter reduction at plug nose is high. Therefore, the frictional force of the round billet in the rotating direction increases, and the wear amount at a part of the surface of the inclined rolls initially contacted to the round billet particularly increases. The wearing of the inclined rolls not only lowers the biting property but also gives rise to an outer surface defect.